Two nice pieces were added to my collection of tubes :). Theese are a Russian (Soviet) pentodes GU-81M with the anode dissipation of 450W (600W max 3 minutes).
They have a diameter of 10 cm, height 24cm. They are directly heated. Voltage 12.6 V and current of 10A. Max. anode current is 600 mA, voltage 3000V up
to 6MHz, 24MHz up to 2500V and 1500V up to 50MHz.
Max. peak anode voltage (anode modulation) may be twice that. Minimal guaranteed lifetime is 1000h. Guaranteed power output (up to 25 meters range) 750W, and at
the end of life
675W. Loss of g2 120W, loss of g1 10W. Slope 4,5-6,5 mA / V. Max. bulb temperature of 350 deg. C.
Here you can find the datasheets of theese tubes: datasheet GU-81 (1), datasheet GU-81 (2), datasheet GU-81 (3).